Electrode Site

Electrode 'site' and 'contact' are synonymous. Each site is connected to its corresponding bond pad through a buried conductive trace. The bond pad is connected to an externalized connector through the electrode package. This makes each site individually addressable to the headstage and recording system.

On a silicon probe the sites are precisely positioned on the shank. On a thin-film grid they are precisely positioned on the face of the polyimide substrate that contacts the tissue.

From a practical standpoint, two primary properties of a site determine its recording and/or stimulation characteristics. First, its geometric area determines the tissue volume it samples, its 'region of interest'. For a silicon probe, a relatively small site (177 µm²) records spike activity of nearby neurons; a relatively large site (703 µm²) averages biopotentials over a larger area, and is well indicated for recording local field potentials. Second, its site material is a primary factor in electrical properties. NeuroNexus uses iridium for most silicon probes, SIROF (sputtered iridium oxide film) where low impedance and high charge-injection capacity are required, and platinum for polymer grids and for the SiNAPS probes. Z-Coat, the proprietary NeuroNexus PEDOT coating, lowers site impedance by roughly an order of magnitude and raises charge capacity without changing the site footprint. These materials are well characterized and have excellent electrochemical properties and biocompatibility for neural interfacing (Cogan (2008); Cogan et al. (2009)).